1. Field of the Invention
The present invention relates to a sensor system.
2. Description of the Related Art
A distance measurement sensor of the light projection triangulation type is available which directs pulsed light to an object to be measured and in which a light receiver portion is located at a given base line length from a light projector portion and light reflected from the object is received by the light receiver portion, and the distance to the object to be measured is determined. If this sensor is operated at a high speed continuously, a large amount of electrical power is consumed.
A conceivable method of reducing the power consumed by such sensor is illustrated in FIG. 4, where a general-purpose controller is added to the sensor. This controller controls the operation of the sensor such that it operates slowly and intermittently.
The configuration shown in FIG. 4 is briefly described. The general-purpose controller (hereinafter referred to as CNT) is indicated by numeral 100 and uses a battery E as a power supply and is connected with a distance measurement module (DMM) 200 via terminals T1-T8 and controls the operation of the distance measurement module 200 via these terminals. Let Tp, P1, P2, and P3 be output signals appearing at terminals Tp, P1, P2, and P3, respectively, shown in FIG. 4.
The CNT 100 is now described. The CNT 100 has a low power-consumption power-supply regulator circuit (hereinafter referred to as REG) 103 for providing a low power-consumption power source Vreg1 to a low power-consumption timer circuit (hereinafter referred to as RTC) 101 and to a CPU 102. A power-supply circuit (REG) 104 provides a power source Vreg2 used as a power source to the DMM 200. The RTC 101 intermittently supplies a trigger signal Tp to the CPU 102, which in turn is triggered by the trigger signal Tp and operates in a given manner. Normally, the CPU 102 is in a standby mode in which the CPU consumes only quite a small amount of electrical power. When the signal P1 from the CPU 102 is ON, the power-supply circuit (REG) 104 generates voltage Vreg2. When the signal P1 is OFF, the power-supply circuit (REG) 104 is in a standby mode in which the circuit consumes only a quite small amount of electrical power. The output signal P2 from the CPU 102 turns on or off the switch SW1. When the signal P2 is ON, the switch SW1 permits supply of a voltage to the DMM 200. A resistor R1 and a capacitor C2 together form a power-supply filter circuit 105 for a distance measurement IC (hereinafter referred to as DMIC) 201. Where the time constant is set large and a large current flows through an IRED (infrared-emitting diode) 202, the filter circuit 105 suppresses variations in the electrical power through the DMIC 201. Also shown are a capacitor C1 and a resistor R4.
The DMM 200 is made up of the DMIC 201, the IRED 202, a PSD (position-sensitive detector) 203, resistors R2, R3. transistors Tr1, Tr2, a capacitor Cc, and so on, and operates as a distance measurement sensor. This sensor has an output terminal at which a signal indicating the result of a distance measurement appears. This output terminal is the collector of the transistor Tr2. That is, the output terminal for producing a signal indicating the result of a distance measurement is the output terminal of open collector type.
The operation is next described briefly by referring to FIG. 5, where Tp, P1, Vreg2, P2, Vd, P3, Ir0, Data, and O1 indicate voltage waveforms at terminals Tp, P1, Vreg2, P2, Vd, P3, IR0, Data, and O1, respectively, shown in FIG. 4. In FIG. 5, IR4 indicates the waveform of current at a terminal IR4 shown in FIG. 4. In FIG. 5, CPU indicates the relation between the operation of the CPU shown in FIG. 4 and its standby mode.
The RTC 101 produces the trigger signal Tp at intervals of t00 time. In response to this, the CPU 102 turns on the signal P1 for a period of t10 time and operates the REG 104. After a lapse of t20 time, the operation of the power-supply circuit 104 stabilizes. Then, the signal P2 is turned on, thus turning on the switch SW1. The output voltage Vreg2 from the REG 104 is supplied to the filter circuit 105, whose output voltage Vd is applied as electrical power to the DMM 200.
In response to the signal P2 being turned on, electrical power is supplied to the DMIC 201 within the DMIC 200. Then, the CPU 102 waits for a period of t30 time to stabilize the DMIC 201. The CPU 102 turns on the operating signal P3 for a period of t40 time, thus causing the DMM 200 to start an operation for a distance measurement.
In response to the application of the operating signal P3 that is in an ON state, the DMIC 201 produces an emission signal IR0 to operate the IRED 202 to perform a measurement of a distance. The result is reflected in the state (i.e., ON or OFF) of the transistor Tr2. The result is sent to the CPU 102 from the collector terminal of the transistor Tr2. In particular, if an object to be measured is detected, the transistor Tr2 is turned on, causing the terminal T8 to assume state 0. If no object to be measured is detected, the transistor Tr2 is turned off, making the terminal T8 assume state 1. When the ON state of the signal P3 ends, the CPU 102 turns off the signals P1 and P2 and returns to the standby mode.
In this way, the CNT 100 controls the operation of the DMM 200 to make intermittent the distance measurement operation of the DMM 200.
In the distance measurement sensor as described above, the capacitor Cc or the like is connected with the distance measurement IC (DMIC) 201 to generate a given frequency. A timing signal for the distance measurement operation is created according to the generated frequency. Therefore, depending on the specifications of the distance measurement sensor, the pulse width of the light emitted from the IRED 202 may be increased. For this purpose, this operating frequency f0 may be lowered. That is, the time taken to project light has some interval of time.
Where this is taken into consideration, in order to assure that data obtained by a measurement of a distance is accepted into the CPU 102 by operating the DMM 200 intermittently, the supply of electrical power to the DMM 200 should not be interrupted during the measurement operation, even if the operating speed of the DMM 200 is low and a long time is taken to emit light.
To solve the aforementioned problem, it is necessary to set the time between the instant when the DMM 200 starts an operation for a distance measurement operation and the instant when the power supply is turned off, i.e., the period of the ON state of the signal P3, be set so long that no troubles take place if the operating frequency f0 is lowest. In this case, an allowance is given, and the possibility of occurrence of malfunctions decreases accordingly. However, the amount of wasteful power consumption increases.
In another method, an adjusting means is fitted to the CNT 100. According to the specifications of the DMM 200, the timing at which the CNT 100 accepts measurement data is slowed or the time for which electrical power is supplied to the sensor is set sufficiently long. In this way, an adjustment is necessary. That is, in this case, the CNT 100 must be adjusted according to the actually used operating timing of the DMM 200. If a general-purpose product is used as the CNT 100, each individual product needs adjustments. Furthermore, if the CNT 100 is managed by adjusting it, the operating frequency f0 varies due to variations in capacitance among the individual products used as the capacitor Cc, temperature variations, and variations in characteristics among the individual products used as the DMIC 201. To cope with variations in the time taken to emit light due to variations in the operating frequency f0 as described above, it is necessary to set the period of the ON state of the signal P3 at a sufficiently long time, even if cumber some operations such as adjustments are performed. As a result, a voltage is kept applied to the DMIC 201 in spite of the fact that the distance measurement operation has already ended in practice. Hence, electrical power continues to be wasted.
This drawback is described by referring to FIGS. 6(a) and 6(b). FIG. 6(a) illustrates the operation when the operating frequency f0 is high. FIG. 6(b) illustrates the operation when the operating frequency f0 is low. The time between the leading edge of the signal P3 and the latching time L is set constant and sufficiently long (tL0=tL0′, t30=t30′, t40=t40′) so that data can be latched at L even if the operating frequency f0 varies. After being read into the CPU 102 and processed, data is delivered from the output terminal O1. Then, the signals P2 and P3 are turned off to stop the supply of electrical power. Also, the CPU 102 itself makes a transition from the operation mode to the standby mode. That is, even after the distance measurement operation ends and data derived from the measurement is delivered, a voltage is kept applied to the DMIC 201. In consequence, wasteful power consumption continues.
This problem is not limited to a light projector-type distance measurement sensor. Rather, this problem arises with every sensor means that performs a desired measurement and then produces an output signal corresponding to the results of the measurement. That is, where the time taken to perform a given measurement has some interval of time, electrical power is supplied for a time sufficient to accommodate the interval. This method produces the aforementioned problem.
Many sensors use the open collector output (Tr2) to send data even to a system using a different voltage or to permit a relay or the like to be directly activated. In this scheme, a pull-up resistor R4 is coupled to the electrical power Vreg1 for the CPU 102 to accept data.
This DMM 200 judges whether there is an object to be measured or not. If an object to be measured is detected, the transistor Tr2 that is normally OFF is kept ON. In this configuration, current is kept supplied to the pull-up resistor R4 from the power source Vreg1 during the period t50 between the instant when the signal Tr2 is turned on and the instant when the signal P3 is turned off and during the period t60 between the instant when the period t50 ends and the instant when the voltage vd reaches the cutoff voltage Vth of the distance measurement IC (DMIC) 201, as shown in FIG. 5, for the following reason. Even if the power to the distance measurement module 200 is cut off, the voltage vd applied to the DMIC 201 does not quickly drop to zero because of electric charge stored in the capacitor C2. Rather, the voltage gradually decreases to the cutoff voltage of DMIC 201 because of power consumption by DMM 200. Therefore, the power consumption at the resistor R4 hinders lower power consumption. If the voltage Vd further drops to the cutoff voltage Vth, the state is often maintained for a relatively short time (from on the order of seconds to on the order of minutes). If power is then supplied, the transistor Tr2 may be kept in conduction. Current starts to flow into the pull-up resistor from the application of the power during a period of t70 time. Such a current is important to a low power-consumption sensor. This hinders reducing the power consumption.